Average annual precipitation for the Niangua Watershed is 40-42 inches per year (MDNR, 1986). The mean monthly precipitation at the Buffalo weather station, which is located near the center of the watershed, is shown in Figure Hy01 The wettest months are typically May, June, and September and the driest are December, January, and February.
Gaging and Water Quality Stations
Gaging and water quality stations are listed in Table Hy01 and mapped in Figure Hy02. There have been six (United States Geological Survey) gaging stations and two low flow stations in the Niangua Watershed. In addition, 19 water quality stations and one gaging station were monitored by the USGS for the Upper Niangua Animal Waste Project (UNAWP) between 1991 and 1995. Five water quality stations were monitored in 1989 and 1990 by a private contractor, Environmental Science and Engineering (ESE), to satisfy Federal Energy Regulatory Commission (FERC) requirements for Tunnel Dam relicensing (ESE, 1990). Sho-Me Electric Corporation helped fund the installation and maintenance of a new gaging station in November 1995 immediately below Tunnel Dam (NR) to monitor flow.
The most downstream USGS station on the NR near Decaturville (G026) indicated a median flow of 325 cfs between 1929 and 1969. The drainage area for this station is 627 square miles. A flow duration curve for the Decaturville station is shown in Figure Hy03. The low 10:90 ratio (ratio of the discharge exceeded 10% of the period of record to that exceeded 90% of the period of record) of 8.8 indicates that flows are not highly variable. This value is at the low end of the range exhibited at other stations with similar drainage areas throughout the state (Skelton, 1976). Although no quantitative data is available, the median flow of the LNR as it enters the lake is usually noticeably less than that of the NR. The most downstream station on the LNR, near Macks Creek (G025), was operated as a low flow and crest station between 1962 and 1971 so median flow is not available. The only continuous record station in the LNR watershed was on Starks Creek (G024), a third order tributary. The Starks Creek flow duration curve (Figure Hy04) with a 10:90 ratio over 400 indicates highly variable flows at this station. This station is in the headwaters of Starks Creek (SM 12) where the average gradient is 30.8 feet per mile.
The magnitude and frequency of low-flows at several stations in the watershed are shown in Table Hy03. The low flow is the lowest average flow over a 7-day period that is likely to occur during a given recurrence interval. These can be useful for evaluating the impacts of effluent discharges or water withdrawals and droughts during critical periods of low flow. In Missouri low flows usually occur during August, September, and October (Skelton, 1976). Skelton also explained that Ozark streams usually have the best sustained low flows in the state, due to large underground reservoirs in the solution dissolved carbonate bedrock. However, solution channels can also divert groundwater before it reaches streams, and drain surface water from losing streams in some areas. In many of the watershed’s streams, considerable water flows in the gravel beneath the stream bed during drought. Fourth order and larger reaches of most tributaries sustain permanent flow throughout the year. Maximum and minimum discharges for four gaging stations are shown in Table Hy03. Small streams in the watershed are flashy, with high flows after significant rainfall. Flood discharges at gaging stations with sufficient data are shown in Table Hy04.
The Niangua Watershed contains numerous springs (Table Hy05; Figure Hy05). Some of the springs listed in Table 12 were found in historical records (Skinner, 1979) or on 7.5 minute topographic maps so their current status is unknown. Skinner (1979) reported that many strong flowing springs went dry following agricultural development in the watershed. Some landowners have also reported that small permanent springs have ceased flowing in the last 50 years (Bob Schulz (MDC), pers. comm.). The largest springs in the watershed are Bennett Spring, the fourth largest in Missouri, and Ha Ha Tonka Spring, the twelfth largest in the state (Vineyard and Feder, 1982). Bennett Spring practically doubles the flow where it joins the NR at SM 65.9. Bennett Spring is supplied by an extensive recharge area (Table Hy04) which has recently been delineated by an MDNR study (Vandike, 1992). The recharge area includes portions of the Dry Glaize and Gasconade watersheds (Vandike, 1992). Ha Ha Tonka Spring flows about 1.4 miles to a cove on the Niangua Arm of LOZ. Many karst features and the dramatic faults evident in the vicinity of Ha Ha Tonka Spring suggest that a large underground reservoir may supply the spring (Vineyard and Feder, 1982). In response to concerns about steady increases in nitrates and phosphates in the late 1960s, a thorough study of potential contamination sources in the vicinity of Ha Ha Tonka Spring was conducted (Vineyard and Feder, 1982). To eliminate pollution sources in the vicinity, several nearby resorts were purchased and an extensive sewer system was installed. Several other springs of the Niangua Watershed are hydrologically connected to losing streams outside the watershed (Figure Hy05).
Nineteen losing streams have been delineated in the Niangua Watershed (Table Hy06; Figure Hy05). A losing stream is a stream segment that loses 30% or more of its flow through permeable geologic materials into a bedrock aquifer. Low flow measurements or dye tracings are used to identify losing stream segments, and the MDNR Water Pollution Control Program maintains a list of identified segments. Wastewater discharges within two miles upstream of a losing stream must meet more stringent effluent limitations due to the potential for groundwater pollution. Thirty additional stream segments within the watershed have been identified as losing streams in recent dye tracings (Vandike, 1992), and are awaiting approval for the MDNR list. In addition, several losing streams have been identified in the spring recharge area that lies outside the watershed (Figure Hy05).
Impoundments are shown in Figure Hy06. Most of these were included in a database maintained by the MDNR. Impoundments with dams over 35 feet high are required to obtain a permit. However, many of the impoundments recorded in the MDNR database are not that large, and were registered voluntarily. Several additional impoundments over ten acres were located on 7.5 minute topographic maps. Lake Niangua (L010) is the largest impoundment in the watershed (360 acres).
Dam and Hydropower Influences
Two hydropower projects impact the Niangua Watershed. Bagnell Dam was completed in 1931 on the Osage River approximately 31 miles downstream from the mouth of the NR. It is owned and operated by Union Electric Company of St. Louis, MO. The facility has eight turbines with a maximum generating capacity of 215,000 kilowatt (kw). It is normally run as a peak load facility, meaning most of the power is generated during periods when there is high demand for electricity. Bagnell Dam impounds 55,000-acre LOZ, which includes the lower 21 miles of the NR and lower 10 miles of the LNR. Nearly the entire shoreline of the lake is privately owned. The Niangua and Little Niangua arms are typical of much of the rest of the lake - highly developed with numerous private dwellings and recreational businesses. Because the lake was constructed primarily for hydropower production rather than flood control the magnitude of water level fluctuations is much less than that of nearby COE lakes constructed primarily for flood control. Detailed information regarding LOZ can be obtained from the Lake of the Ozarks Management Plan (Stoner, 1999).
Tunnel Dam was completed in 1929 on the NR (~SM 29) creating 360 acre Lake Niangua, a very shallow impoundment which extends upstream 2.3 miles. The storage capacity of Lake Niangua is 2,650 acre-feet at normal pool elevation (711.5 feet MSL). The watershed of the reservoir is approximately 600 square miles. The project was originally operated by the Missouri Electric Power Company, but Sho-Me Power Corporation of Marshfield, MO, purchased the facility in 1944. The project is licensed by the Federal Energy Regulatory Commission (FERC). The facility has two turbines with a maximum generating capacity of 2,650 kw. It is a run-of-the-river facility and derives head for generation by diverting river flow from Lake Niangua through a tunnel to the power plant. This diversion results in greatly reduced flow in the bypass reach, approximately 6.5 miles of river between the dam and the powerhouse.
The Tunnel Dam project was recently relicensed for 30 years beginning June 1, 1994 by the FERC (1994). Requirements of the relicensing include:
- Minimum flows are to be released in the by-pass reach as follows: 60 cfs during March 15 - June 15; 40 cfs the rest of the year, or natural inflow, whichever is less.
- The project will continue to operate run-of-river, but Sho-Me Power Corp. has authorization to operate in a peaking mode under the following conditions: peaking can only occur in July and August; it cannot exceed two hours per day; fluctuations in the reservoir surface elevation cannot exceed 0.5 feet; and the resource agencies must be notified.
- A continuous-monitoring gage recorder is required in the bypass reach and the sluice gate will be calibrated to indicate discharge level.
In November 1995, the USGS installed a continuous record gage below Tunnel Dam with financial support from Sho-Me Power Corporation. Provisional data supplied by the USGS indicated that between December 5, 1995 and December 4, 1996, the daily mean discharge was below the required minimum on 111 of 356 days. The minimum mean daily discharge of 28 cfs was recorded on two separate days. The measured discharge was below the minimum flow requirement (60 cfs) during the spawning season (March 15 to June 15) on 51 of 93 days in 1996.